This invention relates to a photothermographic material capable of forming an image through heat development and more particularly, to a photothermographic material having a non-photosensitive layer on the back surface, that is, a back layer which can be coated at a low cost without a need for a harmful organic solvent. The photothermographic material is often referred to as a photosensitive material.
There are known many photosensitive materials comprising a photosensitive layer on a support which are exposed imagewise to form images. Among them, a process of forming an image through heat development is known as an environment friendly system capable of simplifying image forming means.
The process of forming an image through heat development is disclosed, for example, in U.S. Pat. No. 3,152,904 and 3,457,075, and D. Morgan and B. Shely, "Thermally Processed Silver Systems" in "Imaging Processes and Materials," Neblette, 8th Ed., Sturge, V. Walworth and A. Shepp Ed., page 2, 1969. These photosensitive materials generally contain a reducible non-photosensitive silver source (e.g., organic silver salt), a catalytic amount of a photocatalyst (e.g., silver halide), and a reducing agent for silver, typically dispersed in an (organic) binder matrix. Photosensitive materials are stable at room temperature. When they are heated at an elevated temperature (e.g., 80.degree. C. or higher) after exposure, redox reaction takes place between the reducible silver source (functioning as an oxidizing agent) and the reducing agent to form silver. This redox reaction is promoted by the catalysis of a latent image produced by exposure. Silver formed by reaction of the organic silver salt in exposed regions provides black images in contrast to unexposed regions, forming a black and white image.
In conjunction with photothermographic materials, it is well known to form a back layer by applying a coating solution of a binder in an aqueous solvent and drying the coating. Such a back layer is referred to as an aqueous back layer, hereinafter. For example, JP-A 254443/1990 discloses the use of gelatin as the binder and JP-A 129220/1976 discloses the use of polyvinyl alcohol as the binder.
As compared with back layers formed by applying a coating solution of a binder in an organic solvent and drying the coating, the aqueous back layers have environmental and economical advantages that they eliminate the detrimental influence of organic solvents on the environment and human body as well as the recovery of organic solvents.
Photosensitive materials having such aqueous back layers, however, suffer from drawbacks that when sheets of photosensitive material are stored in a humid atmosphere, fog increases and sheets stick to each other. It is then desirable to provide a photosensitive material with a back layer which is free of a fog increase, a sticking phenomenon, and detrimental influence on the environment and human body and is economically advantageous.